In his new book, The Way of the World, Ron Suskind makes the following claims:

• MI6, Britain's secret intelligence service, told Tony Blair before the invasion of Iraq that Saddam Hussein did not have any weapons of mass destruction. The intelligence was passed to the US but the White House buried it.

• At the beginning of 2003, weeks before the invasion of Iraq, MI6 sent Michael Shipster, one of its senior officers, to Amman, the Jordanian capital, to meet Tahir Jalil Habbush, the head of Iraqi intelligence. Sir Richard Dearlove, then head of MI6, described the secret mission as an "attempt to try, as it were, I'd say, to defuse the whole situation". He said the "Cheney crowd" was in too much of a hurry and Bush did not resist them strongly enough.

• Nigel Inkster, a former MI6 officer, confirmed that Habbush told Shipster there were no banned weapons in Iraq. Rob Richer, a former CIA officer, said Britain wanted to avoid a war, but Bush wanted one.

The American Pulitzer prize-winning author has got things wrong in the past and it is extremely difficult in the spooky world to get at the truth. Sometimes, the best criterion – at least, before more evidence emerges confirming or denying the claims – is plausibility. Unless convincing rebuttals emerge soon, Suskind will be given the benefit of the doubt.

In the run-up to war, senior British security and intelligence officials made it clear – privately – that they were strongly opposed to the invasion of Iraq. Dearlove, we know from a leaked Downing Street minute, warned Blair in July 2002 after he returned from a visit to Washington, that the "intelligence and facts were being fixed around the policy". Unfortunately, those officials did not have the courage to say what they thought and believed, if necessary if it meant that they would have to resign. On the contrary, as the Hutton inquiry and the Butler report so damningly demonstrated, they succumbed to political pressure, allowing Blair to go along with Bush's war.

The Butler report savaged the government's now-discredited Iraqi weaspons dossier. It also described in detail how MI6 placed far too much reliance on claims by its agents that Saddam had amassed weapons of mass destruction than it should have and appeared to ignore contrary evidence, such as that now claimed to have come from Habbush.

"Ultimately, Habbush could not offer proof that weapons that didn't exist, didn't exist," Suskind writes. George Tenet, CIA director at the time, claims that Habbush had "failed to persuade" the British that he had "anything new to offer by way of intelligence".

Tenet said in a statement: "There were many Iraqi officials who said both publicly and privately that Iraq had no WMD – but our foreign intelligence colleagues and we assessed that these individuals were parroting the Ba'ath party line and trying to delay any coalition attack. The particular source that Suskind cites offered no evidence to back up his assertion and acted in an evasive and unconvincing manner."

Dearlove complained last year that the Blair government placed too much weight on intelligence claims in order to help persuade opponents in parliament to support the war. That was a cop-out.

If Habbush was talking to MI6, then who better as a source? If MI6 – and the CIA – did not believe him, who else did they believe? And why? What is the point of having intelligence agencies? After the invasion, Habbush was paid $5m by the CIA for serving as an informant and resettled in Jordan. According to Suskind, White House officials susbsequently used him to help with a forgery.

In September 2003, Suskind writes, the White House directed Tenet to concoct a fake letter, backdated to July 2001 but bearing Habbush's signature, claiming that Mohamed Atta, leader of the September 11 hijackers, had been trained in Iraq for the mission. Habbush agreed to sign the letter, which was then leaked to a the Sunday Telegraph.

We already know that Cheney and Co put it about, and persuaded some CIA officers to agree, that there was a link between al-Qaida and Saddam Hussein, a claim which a moment's thought would conclude was preposterous. Some British journalists bought the claim but at least, on this issue, MI6 did its best to quash it.

Postscript (August 7 2008, 18:30): Since this article was first posted (on August 6 2008), I have been able to talk to Nigel Inkster. After reading the comments attributed to him in Suskind's book, he could only describe them as "inaccurate and misleading". Inkster says:

"Mr Suskind appears to have conflated separate conversations; one about the problems of reading Saddam Hussein's intentions, an issue which is dealt with in the Butler report, and one about Habbush. I made it clear to Mr Suskind that I was in no position to comment on the substance or significance of any dealings with the latter since I had not been privy to the detail of what had taken place, something Mr Suskind has chosen not to mention. And, in any event, I had made it clear to Mr Suskind, when first he approached me, that I would not divulge classified information to which I had had access during my time in government.

"Mr Suskind's characterisation of our meeting is more the stuff of creative fiction than serious reportage, and seeks to make more of it than the circumstances or the content warranted."